1. Technical Field of the Invention
The present invention relates to a telescopically moving structure comprising a nut mounted in a rotationally fixed manner in relation to a frame part, a spindle tube with an eternal thread in engagement with the nut, which spindle tube furthermore has an internal thread with the opposite direction to that of the external thread, which internal thread is in engagement with a non-rotational spindle provided with an external thread, the spindle tube being connected in a rotationally fixed manner, but in the axial direction in a slidable manner to a driving shaft.
2. Prior Art
Telescopically moving structures of this kind have an increasing importance as demands for compact and versatile telescopic structures are growing. Structures of said kind can, as will appear later, be telescoped to essentially the triple length, which makes them very suitable for e.g. height-adjustment of tables, however, they are used in several connections.
For many purposes it is necessary or at least preferable that the means for rotation of the spindle tube, and thus for providing the telescopic movement, are situated at one end of the structure, and that the means for rotation of the spindle tube are stationary in relation to this end of the spindle tube. Further it is an advantage that the means for rotation of the spindle tube in the axial direction are connected in a slidable manner to the tube such that a load in the axial direction is not transferred to the driving mechanism. This means that gearwheels etc. do not have to be dimensioned in order to stand a blow in axial direction. If, for example, a height-adjustable table during transport or the like is dropped from a few centimeters, the driving mechanism can easily be damaged, if this axial force is transmitted to gear wheels or the like. Therefor height-adjustable tables must be dimensioned to withstand a fall according to a so-called drop-test, but a telescopically moving structure connected in a slidable manner to the drive system merely has to be dimensioned to endure the forces at retracting or telescoping the structure, which means, for example, that the structure can be designed with nylon gearwheels which are much cheaper than steel gearwheel.
U.S. Pat. No. 3,404,580 discloses an actuator in which a first threaded spindle tube is driven to rotate by a splined shaft slidably engaged in a splined hole in one end of the spindle tube. The spindle tube is engaged in a nonrotary nut, and a second, nonrotary spindle tube is threadably engaged in an internal thread of the first spindle tube, the internal and external threadings of the first spindle tube being of different directions.
A disadvantage of this actuator is that the spindle tube is driven by a small diameter splined shaft, which will not be able to transfer high torque because this may result in seizure so the splined shaft cannot slide in the splined hole.
Similarly DE 39 10 814 A1 discloses a linear actuator in which a first, threaded spindle tube is driven to rotate by a splined shaft slidably engaged in a splined hole in one end of the spindle tube. The spindle tube is engaged in a nonrotary nut, and a second, nonrotary spindle tube is threadably engaged in an internal thread of the first spindle tube, the internal and external threadings of the first spindle tube being of different directions.
A disadvantage of this actuator is that a splined shaft of small diameter will not be able to transfer high torque, because this may result in seizure so the splined shaft cannot slide in the splined hole.
U.S. Pat. No. 1,708,450 discloses a dental chair having an adjustable support in which a first spindle tube is driven to rotate, so a nonrotary nut engaged thereon translates along the spindle tube. A nonrotary spindle is engaged in an internal thread of the first spindle, and a system of rollers prevents the different nonrotary parts from rotating.
A disadvantage of this construction is that it is rather complicated and has large dimensions, so it is not suitable for being used as a telescopically moving structure in general, and further the drive means is not slidably connected to the spindle.